Mossy Fibers¶
Overview¶
Adapted from: https://en.wikipedia.org/wiki/Mossy_fiber_(cerebellum):
Mossy fibers are one of the major inputs to cerebellum. There are many sources of this pathway, the largest of which is the cerebral cortex, which sends input to the cerebellum via the pontocerebellar pathway. Other contributors include the vestibular nerve and nuclei, the spinal cord, the reticular formation, and feedback from deep cerebellar nuclei. Axons enter the cerebellum via the middle and inferior cerebellar peduncles, where some branch to make contact with deep cerebellar nuclei. They ascend into the white matter of the cerebellum, where each axon branches to innervate granule cells in several cerebellar folia.
See Microcircuit of Cerebellar cortex for a diagram of mossy fibers.
Quantity¶
In cat:
So far, I have not found an explicit estimate of the total number of mossy fibers in cat. Instead, the numerical values provided are based on the divergence and convergence between cell types within a folium. The unknown factor is the number of folia which are innervated by each mossy fiber. Statements related to the quantity are below.
Following from [ItoM-1984], page 86:
Mossy fiber-Purkinje cell ratio within the folia is 4:1 [PalkovitsM+2-1972]. If each mossy fiber afferent innervates two folis by branching in the white matter, there will be 2.4 x 10^6 mossy fibers in the whole cat cerebellum (a total number of Purkinje cells is assumed to be 1.2 x 10^6; [PalkovitsM+2-1971a]. However, this is an overestimate if the mossy fibers branch more abundantly.
“According to earlier data [PalkovitsM+2-1971b] the granule cell-glomerulus ratio is 27-28:1, the mossy fiber-granule cell ratio is therefore 1:460.” [PalkovitsM+2-1972].
The granular layer contributed to 29.09% of the total cerebellar volume, their absolute number being 2.2 x 10^9. [PalkovitsM+2-1971b] p. 29.
The above two would mean 4.78 x 10^6 mossy fibers (2.2 x 10^9 granule cells / 460);
Structure¶
“Within a folium, a mossy fiber branches along the plane perpendicular to the long axis of the folium. Therefore, the cascade arborization of mossy fibers (Cajal, 1911) tends to be parallel with the dendritic arborization of Purkinje cells.” [ItoM-1984], p. 87.
Connection to glomeruli¶
Divergence¶
“One mossy fiber breaks up (within a given folium) into about 16-17 mossy rosettes” (which glomeruli form around). [PalkovitsM+2-1972]. From [ItoM-1984], page 86: “If each mossy fiber afferent innervates two folis by branching in the white matter, …” (Comment:: Would this mean about 32 glomeruli per mossy fiber)?
Convergence¶
1? (Comment: I think each glomeruli is associated with just one mossy fiber rosette. However, in a table of properties used for a computational model ([DAngeloE+11-2016] Table 2) it says that both the convergence and divergence from mossy fibers to glomeruli is “not known” (row 2 of Table 2)).
Connection to grannule cells¶
Divergence¶
- Cat:
1.7 x 10^3 (from [LoebnerEE-1989], fig 2).
From [PalkovitsM+2-1972], p. 26: “Four mossy fibers entering a folium give rise to 16 rosettes each, hence a total of 64 glomeruli. Since one glomerulus has synaptic contacts with an average of 28 granule cells, the total number of granule cells reached by the 4 mossy fibers will be 1,792. Each granule cell is presumed to pick up excitatory impulses from 4 glomeruli belonging to different mossy fibers by as many dendrites.”
but also, from the same paper, page 28: “The granule cells have 4.17 dendrites, on average; the average mossy rosette is contacted by 112 granule dendrites. The number of postsynaptic units (dendrite digits) is 10.2/dendrite and 1,142/glomerulus.”
At first, this seems contradictory, (28 granule cells vs 112 granule dendrites per glomerulus). As described in the section of Golomeruli to grannule connection Connection to grannule cells I think the fan out is 1,792 per mossy fiber (as given in [LoebnerEE-1989], fig 2).
Convergence¶
Cat:
4 (from [LoebnerEE-1989], fig 2); 4.17 (from [PalkovitsM+2-1972], p. 28).
Mouse and in general:
About 4. Analysis of why 4 is optimal in: [BillingsG+4-2014].
Connection to Golgi cells¶
Divergence¶
Cat:
Unknown ([LoebnerEE-1989], fig 2).
Convergence¶
Cat:
Unknown ([LoebnerEE-1989], fig 2).
Connection to DCN (Deep Cerebellar Nuclei)¶
Divergence¶
Cat:
Unknown ([LoebnerEE-1989], fig 2).
Convergence¶
Cat:
Unknown ([LoebnerEE-1989], fig 2).